There has been developed and has come into practical use a vehicle on which a powertrain, which is called a hybrid system wherein an engine (for example, a well known engine such as a gasoline engine or a diesel engine is considered to be used) and an electric motor are combined, is mounded. Such a vehicle is controlled so as to achieve the most efficient performance by automatically switching between operation by the engine and operation by the electric motor regardless of an accelerator operation amount by a driver. For example, in the case where the engine is operated in a steady state and operated for rotating a generator that charges a secondary battery (battery) acting as an energy storage mechanism, in the case where the engine is intermittently operated while the vehicle is running according to an amount of charge, or the like, the operation and stop of the engine are repeated regardless of the accelerator operation amount. In other words, the engine and the electric motor are separately operated or cooperatively operated, thereby enabling the improvement in fuel consumption and the great suppression of exhaust gas.
As this secondary battery, a high-voltage nickel hydride battery or the like is used for supplying power to the electric motor. Additionally, such a vehicle mounted with the hybrid system is provided with a DC/DC converter for charging an auxiliary battery or an EPS (Electric Power Steering) that receives a supply of the power from this secondary battery, instead of receiving a supply of the power from a conventional auxiliary battery. In other words, such a vehicle is mounted with a plurality of high voltage electrical equipment, to which the power from the high voltage secondary battery is supplied.
Japanese Patent Laying-Open No. 2004-72892 discloses an electrical load driving device capable of operating a part of a plurality of electrical loads even when a different part of the plurality of electrical loads is out of control. This electrical load driving device includes a power source that outputs a DC voltage, a voltage converter that changes a voltage level of the DC voltage to output an output voltage, a first electrical load that is driven by the output voltage outputted from the voltage converter, a second electrical load connected between the power source and the voltage converter, and control means for stopping the voltage converter when a malfunction occurs in the first electrical load.
According to this electrical load driving device, the driving device includes the control means that controls so as to stop a boost converter (voltage converter) that supplies the DC voltage to an inverter driving an AC motor and to continue to supply a DC voltage of the DC power source to an auxiliary system (second electrical load), for example when the AC motor as the first electrical load is out of control and a charging amount of the DC power source reaches full charging amount. Accordingly, even if the main electrical load is out of control, the other electrical load can be operated without interruption.
In Japanese Patent Laying-Open No. 2004-72892 described above, however, the boost converter is stopped and the supply of the power to the auxiliary system is maintained while continuing on an ON state of a system main relay, assuming the case where the control of stopping the boost converter can not be performed, it is necessary to protect the system by shutting off the system main relay. Thus, the shutting off the system main relay causes the stop of the power supply to the electrical load. At this time, when the power supply to the electrical load is stopped, the actuation of the electrical load (for example, EPS) is abruptly stopped. In such a case, a driver feels uncomfortable.